Solar cells produce electric current and voltage in order to power an external load, and a primary goal of solar cell design is to increase power output while balancing manufacturing cost. Due to the widespread availability and low cost of silicon versus other semiconductor materials, it has remained the overwhelming choice for solar cell manufacturers. However, since silicon is an “indirect band gap” semiconductor, energy (heat) must be exchanged with the crystal lattice in the form of phonons in order to free electrons. Silicon only uses a portion of the solar spectrum to free electrons. Much of the remaining spectrum energy is absorbed by the crystal lattice, which causes the temperature of the solar cell to rise during normal operation. Additionally, the low surface-state density characteristic of silicon makes it susceptible to radiation damage over time, especially in outer-space applications. High energy particles from the sun create intermediate energy states in a solar cell which lead to higher recombination rates and lower efficiency.
Surface-state density of Gallium Arsenide (GaAs) is much larger than silicon, and the material is inherently harder to total-dose radiation. See Kerns et al., “The design of radiation-hardened ICs for space: a compendium of approaches,” Proceedings of the IEEE 76(11) (1988). GaAs is a “direct band gap” semiconductor that absorbs photon energy and free electrons without transferring momentum, and less heat is absorbed in the crystal lattice. This generates significant improvements for solar cell design such as lower operating temperatures in a given environment. See Silverman et al., “Outdoor performance of a thin-film gallium-arsenide photovoltaic module,” Proc. IEEE Photovoltaic Specialist Conference (2013). GaAs provides additional advantages over silicon including thinner absorbing layers, which improves flexibility and reduces weight. Additionally, GaAs cells maintain performance advantages as irradiance decreases. Generally, high-efficiency GaAs cells produce about 20% more power than high-efficiency silicon cells at room temperature, and about 28% more power at typical operating temperatures. See Reich et al., “Weak light performance and spectral response of different solar cell types,” Proc. 20th European Photovoltaic Solar Energy Conference (2005).
Further advantages in solar cell operation may accrue through the placement of all electrical contacts on the back-surface of the solar cell, which improves both the optical and electrical performance of the solar cell since shading is eliminated and robust electrical contacts may be used to decrease serial resistance. To date, back-surface contact solar cell designs have focused almost exclusively on silicon as the semiconductor of choice.
It would be advantageous to provide a GaAs-based solar cell having relatively optimized layer structure and doping concentrations for back-surface contact operation. Such a solar cell would provide significant advantage over silicon-based cells while additionally providing the advantages associated with back-surface contact placement. It would also be advantageous to provide a GaAs-based solar cell that does not have an emitter layer. Such a solar cell would be easier and cheaper to manufacture while additionally proving performance advantages.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.